Briefly, in the semiconductor industry, microlithography (or simply lithography) is the process of printing the circuit patterns on a semiconductor wafer (for example, a silicon or GaAs wafer). Currently, optical lithography is the predominant technology used in volume semiconductor manufacturing. Such lithography employs light in the visible to deep ultraviolet spectrum range to expose the resist on wafer. In the future, extreme ultraviolet (EUV) and soft x-rays may be employed. Following exposure, the resist is developed to yield a relief image.
In optical lithography, a photomask (often called mask or reticle) is first written using electron-beam or laser-beam direct-write tools. A typical photomask for optical lithography consists of a glass (or quartz) plate of six to eight inches on a side, with one surface coated with a thin metal layer (for example, chrome) of a thickness of about 100 nm. The chip pattern is etched into the metal layer, hence allowing light to transmit through. The area where the metal layer is not etched away blocks light transmission. In this way, a pattern may be projected onto a semiconductor wafer.
The photomask contains certain patterns and features that are used to create desired circuit patterns on a wafer. The tool used in projecting the mask image onto wafer is called a stepper or scanner (hereinafter collectively called “photolithographic equipment”, “scanner”, or “stepper”). With reference to FIG. 1, the block diagram schematic of an optical projection lithographic system 10 of a conventional stepper includes an illumination source 12, an illumination pupil filter 14, lens subsystem 16a-c, mask 18, projection pupil filter 20, and wafer 22 on which the aerial image of mask 18 is projected.
With reference to FIG. 1, by way of background, illumination source 12 may be laser source operated, for example, at UV (ultra-violet) or DUV (deep ultra-violet) wavelengths. The light beam is been expanded and scrambled before it is incident on illumination pupil 14. The illumination pupil 14 may be a simple round aperture, or have specifically designed shapes for off-axis illumination. Off-axis illumination may include, for example, annular illumination (i.e., the pupil is a ring with a designed inner and outer radius), quadruple illumination (i.e., the pupil has four openings in the four quadrant of the pupil plane), and other shapes like dipole illumination. FIGS. 2A and 2B illustrate exemplary annular and quadruple illumination, respectively.
With continued reference to FIG. 1, after illumination pupil 14, the light passes through the illumination optics (for example, lens subsystem 16a) and is incident on photomask (or mask) 18. The mask 18 contains the circuit pattern to be imaged on wafer 22 by the projection optics. As the desired pattern size on wafer 22 becomes smaller and smaller, and that pattern becomes closer and closer to each other, the lithography process becomes more challenging. In an effort to improve imaging quality, current processing techniques employ resolution enhancement technologies (“RET”), such as, for example, optical proximity correction (“OPC”), phase shift masks (“PSM”), off-axis illumination (“OAI”), condenser and exit pupil filters, and techniques applying multi-level illumination (for example, FLEX).
Many of the RET technologies are applied on or directly to mask 18. For example, OPC and PSM, which modify the light wave to (1) compensate the imperfection of the imaging property of the projection optics, for example, the OPC technology is used to compensate the optical proximity effect due to light interference, and/or (2) take advantage of designed light interferences to enhance the imaging quality, for example, the phase shift mask technology is used to create phase shifting between neighboring patterns to enhance resolution.
Notably, mask 18 may not be “perfect”, due to its own manufacturing process. For example, corners on mask 18 may not be sharp but may be rounded; and/or the linewidth may have a bias from design value where the bias may also depend on the designed linewidth value and neighboring patterns. These imperfections on mask 18 may affect the final imaging quality.
The projection optics (for example, lens subsystems 16b and 16c, and projection pupil filer 20) images mask 18 onto wafer 22. In this regard, the projection optics includes a projection pupil filter 20. The pupil 20 limits the maximum spatial frequency of the mask pattern that can be passed through the projection optics system. A number called “numerical aperture” or NA often characterizes pupil 20. There are also proposed RET techniques that modify pupil 20, which is generally called pupil filtering. Pupil filtering may include modulation for both the amplitude and the phase on the passing light beams.
Due to the wavelength of light being finite, and current techniques employing wavelengths that are larger than the minimum linewidth that is printed on wafer 22, there are typically significant light interference and diffractions during the imaging process. The imaging process is not a perfect replication of the pattern on mask 18. Current techniques employ physical theory to model this imaging process. Further, due to the high NA value of current lithography tools, different polarizations of the light provide different imaging property. To more accurately model this process, a vector-based model may be used.
The projection optics may be diffraction-limited. However, lens subsystem 16c in the projection optics is most often not completely “perfect”. These imperfections may be modeled as aberrations, which are often abstracted as some undesired phase modulation at the pupil plane, and are often represented by a set of Zernike coefficients. After the light finally reaches the surface of wafer 22, they will further interact with the coatings on wafer 22 (for example, the photo-resist). In this regard, different resist thickness, different optical properties of the resist (for example, its refractive index), and different material stack under the resist (for example, bottom-anti-reflection-coating or BARC), may further affect the imaging characteristics itself. Some of these effects may also be abstracted by a modulation at the pupil plane.
When the resist is exposed by the image and thereafter baked and developed, the resist tends to undergo complex chemical and physical changes. First principle and empirical models have been developed to simulate these processes.
To design and evaluate the specific implementations of the mask, including a mask implementing RET, and to assess the impact on the quality of the printed pattern on wafer from the RET design combined with the stepper settings and characteristics, computer simulations have been employed to imitate the anticipated and/or expected results. Notably, physical models have been developed for nearly every step of the lithography process, including mask making, stepper's imaging path from illumination to on-wafer image, and the resist exposure and development.
Currently, there are a number of computer software techniques that address needs in lithography simulation. For example, there is first-principle-modeling-based simulation software that conducts detailed simulation of the physical and chemical processes, but runs extremely slow and hence limited to extremely small area of chip design (on the order of a few square microns), for example, “SOLID-C” from Sigma-C (Campbell, Calif., USA) and “Prolith” from KLA-Tencor (San Jose, Calif., USA). Although there is computer software that executes and provides simulation results faster, such software uses empirical models that are calibrated to the experimental data (for example, “Calibre” from Mentor-Graphics, Wilsonville, Oreg., USA). Even for the “fast” simulation that uses empirical model, a simulation at a full-chip level often requires tens of hours to many days.
Moreover, to more fully understand, design, analyze and/or predict the lithography process, the entire process, from illumination—to mask—to imaging—to resist, should, or may need to be analyzed and/or simulated. Due to the complex models and the large amount of design data (today's VLSI design data can reach tens of GB per layer), brute-force computation on general-purpose microprocessors tends to be unwieldy and time-consuming. Further, employing highly specialized mainframe computers would likely require an extensive investment thereby making the process uneconomical.
There is a need for a system and technique that accelerates lithography simulation, inspection, characterization and/or evaluation of the optical characteristics and/or properties, as well as the effects and/or interactions of lithographic systems and processing techniques that overcome one, some or all of the shortcomings of the conventional systems and approaches. There is a need for a system and technique that facilitates verification, characterization and/or inspection of RET designs, including detailed simulation of the entire lithography process to verify that the RET design achieves and/or provides the desired results on final wafer pattern.
Moreover, there exists a need for a system and technique that rapidly simulates, characterizes, inspects, verifies and/or enables RET designs and photolithographic equipment optimization and processes (for example, critical dimension (“CD”, i.e., linewidth of the critical lines in the integrated circuit design), line-end pullback, edge placement error for one, some or every pattern at one, some or every location, and/or printing sensitivity to process variations such as mask error, focus, dose, numerical aperture, illumination aperture and/or aberration).